A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus includes a projection system to project a patterned radiation beam onto the substrate. The projection system may include optical elements, such as mirrors or lenses. The projection system provides for a projection of the patterned radiation beam onto a target portion of the substrate. As accuracy and resolution of the pattern to be projected onto the substrate increases year after year, requirements on accuracy of the projection system increase too. A difficulty associated with a conventional projection system is that a position of the output beam of the projection system and thus a position of an image with respect to the substrate shows a tolerance due to, e.g., vibrations in the lithographic apparatus, tolerances of optical components or other components in the projection system, temperature deviations causing thermal effects such as expansion, drift of one or more sensors involved, and other causes. Further factors influencing a position of the output beam of the projection system include positioning errors of a light source or positioning errors of the patterning device which also may result in a positioning error of the output beam of the projection system.
One way to compensate for such errors could be by a displacement of the substrate, thus displacing the substrate to compensate for a position error of the output beam of the projection system irradiating (a part of) the substrate. However, such a solution may only be partly effective, as a positioning of the substrate to compensate for any position errors in the output beam of the projection system is slow as it requires a displacement of the substrate as well as the substrate table by which the substrate is held, and this solution being associated with a positioning (thus acceleration and deceleration) of a part having a considerable mass.